Conventionally, there have been known spark plugs that have a center electrode and a ground electrode opposed to each other, and cause a spark discharge to occur through application of a voltage between the center and ground electrodes. Moreover, it has always been pursued to extend the service lives of the spark plugs.
In particular, in cases where the center electrode is provided by welding, there exists in the center electrode a fusion portion that contains oxide layers to a great extent. Therefore, as shown in FIG. 4, when a spark discharge occurring from the center electrode is blown to the downstream side by the influence of a gas flow (i.e., a flow of an air-fuel mixture) in a cylinder of an internal combustion engine, the fusion portion is worn down by formation of a cathode point of the spark discharge in the fusion portion, resulting in an early wear of the center electrode.
In addition, as a measure of extending the service lives of the spark plugs, there has been considered a configuration where the dimensions of a noble metal chip provided in the center electrode by welding are increased (see, for example, Patent Document 1). However, in the case of employing this configuration, the cost is increased; in addition, since the fusion portion still exists in the center electrode, the problem of wear of the fusion portion remains unsolved.
Moreover, there has also been known a configuration where both a columnar main chip and an annular auxiliary chip surrounding the main chip are provided in a ground electrode (see, for example, Patent Documents 2 and 3). However, this configuration has been developed for preventing an unintended multiple discharge from occurring due to the blow-out of sparks with increase in the gas flow speed and thereby suppressing wear of the ground electrode. That is, this configuration has been developed not for suppressing wear of the center electrode.